Method for imparting selectivity to otherwise nonselective polymer profile control gels

ABSTRACT

A method for selectively closing pores in a zone of greater permeability in a formation for profile control. A rehealable Xanthan biopolymer is combined with a crosslinked non-selective polyacrylamide polymer gel. The combined gel system is injected into a formation where the Xanthan biopolymer gel selectively enters a zone of greater permeability carrying therewith said non-slective gel. Once in the formation&#39;s zone of greater permeability, the gel reheals and forms a rigid gel with substantially better temperature stability.

FIELD OF THE INVENTION

This invention relates to the use of gels for profile control so thatincreased amounts of hydrocarbonaceous fluids can be obtained from alesser permeability zone in a formation.

BACKGROUND OF THE INVENTION

In the recovery of oil from oil-containing formations, it is usuallypossible to recover only minor portions of the original oil-in-place byso-called primary recovery methods which utilize only natural forces. Toincrease the recovery of oil a variety of supplementary recoverytechniques are employed. These techniques include waterflooding,miscible flooding, and thermal recovery.

A problem that arises in various flooding processes is that differentstrata or zones in the reservoir often posses different permeabilities.Thus, displacing fluids enter high permeability or "thief" zones inpreference to zones of lower permeability. Significant quantities of oilmay be left in zones of lower permeability. To circumvent thisdifficulty the technique of profile control is applied to plug the highpermeability zones with polymeric gels and this divert the displacingfluid into the low permeability, oil rich zones. Among the polymersexamined for improving waterflood conformance are metal-crosslinkedpolysaccharides, metal-crosslinked polyacrylamides, andorganic-crosslinked polyacrylamides.

Basic to the problem of diverting displacing fluid with polymeric gelsis the necessity of placing the polymer where it is needed, i.e. in thehigh permeability zone. This is not difficult if the gel is formed aboveground. Xanthan biopolymers may be crosslinked with metal ions such asCR⁺³ above ground to give gels. These gels are shear thinning and can beinjected into the formation where they then reheal. Since gel particlesare being injected, they will of necessity go into high permeabilityzones. However, many other gel systems are formed in-situ. One systemdisclosed in U.S. Pat. No. 3,557,562 contains acrylamide monomer,methylene-bis-acrylamide as an organic crosslinker, and a free radicalinitiator. This system undergoes polymerization in the formation to givea polyacrylamide crosslinked with methlene-bis-acrylamide. However, theviscosity of the solution when injected is like that of water. Unlessmechanical isolation is used, these solutions are quite capable ofpenetrating low permeability, oil bearing zones. Another form of in-situgelation involves the injection of polyacrylamide containing chromium inthe form of chromate. A reducing agent such as thiourea or sodiumthiosulfate is also injected to reduce the chromate in-situ to Cr⁺³ , aspecies capable of crosslinking hydrolyzed polyacrylamide. Even thoughthe polyacrylamide solution has a viscosity greater than water, it isnot capable of showing the selectivity that a gel can. Thus,polyacrylamides crosslinked with chromium in-situ can also go into lowpermeability zones. It is not useful to crosslink polyacrylamides aboveground and inject them as gels, because polyacrylamide gels undergoshear degradation.

Therefore, what is needed is a method where a shear thinning rehealableex-situ gel can be combined with an in-situ gel so as to obtain greaterselectivity in closing a zone of greater permeability in a formationwhile forming a gel having substantially better qualities to withstandformation conditions.

SUMMARY

This invention is directed to a method for sequential gellation into aformation having varying permeabilities. In the practice of thisinvention, a first gel is placed into an aqueous solution in an amountsufficient to enter pores in a formation's zone of greater permeability.This gel forms ex-situ and is shear thinning. Afterwards, a second gelis formed in-situ. Said second gel is substantially more resistant toformation conditions than said first gel.

After mixing, the aqueous solution containing the gelled ex-situ gel andthe ungelled in-situ gel is directed into the formation's zone ofgreater permeability. Said ex-situ gel selectivity enters pores in thezone of greater permeability. Here it reheals. Thereafter, heat from theformation causes the in-situ gel to firm and form a solid gel which issubstantially more resistant to formation conditions than said firstgel.

It is therefore an object of this invention to make a gel system whereone gel can selectively enter a high permeability zone and reheal whiletransporting a substantially thinner non-selective gel into said highpermeability zone.

It is another object of this invention to keep a thinner in-situ gelfrom entering a zone of lesser permeability.

It is yet another object of this invention to place a substantially lessviscous gel into a formation's zone of greater permeability where it canform in-situ a gel substantially more resistant to formation condition.

It is still another object of this invention to place a substantiallyless viscous gel into a high permeability zone without utilization ofmechanical isolation.

It is a yet still further object of this invention to place apolyacrylamide polymer crosslinked with methylene-bis-acrylamide into alow permeability zone where it can form a gel in-situ.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

Sequential gellation can be accomplished by the inclusion of an ex-situand an in-situ gel into one system. In the preparation of this systemtwo functionally different gels are utilized. One gel is utilized toobtain selectivity so that the combined gel system can enter zones ofgreater permeability in a formation. Another gel is used to obtainincreased rigidity and better temperature stability. Utilization of thecombined system allows it in one sequence to enter a more permeable zoneof the formation. In another sequence, the combined system propagates adesired distance into a formation. Once the system has propagated to thedesired distance into the formation, it forms a rigid gel havingsubstantially better temperature stability.

In one embodiment of this invention, a biopolymer gel is utilized forformation selectivity, and an organic crosslinked polyacrylamide gel isutilized for rigidity and increased thermal stability. The preferredbiopolymer for utilization herein is a Xanthan biopolymer which isobtainable from Pfizer Co. The trademarked product is marketed as Flocon4800C. The polyacrylamide preferred for utilization is a polyacrylamidecrosslinked with methylene-bis-acrylamide.

A typical formulation of Xanthan biopolymer consists of 2000 ppm Flocon4800C, 100 ppm NaOH, and 80 ppm Cr⁺³ . This formulation forms a gel inabout four hours, although gelation starts soon after addition of Cr⁺³ .The four hour gel is capable of being injected into a formation becauseit is shear thinning but it will reheal. The Flocon 4800C gel isreasonably firm with a consistency like that of agar or gelatin. Due togel instability at higher temperatures, the useful temperature limit forFlocon 4800C gels is about 140-150° F.

A typical commercial formulation of polyacrylamide crosslinked withmethylene-bis-acrylamide contains 10% acrylamide and 0.36%methylene-bis-acrylamide. Free radical polymerization is accomplishedwith initiators such as peroxides or azo compounds that decompose atreservoir temperatures. The use of activators such has alkanol aminescan further reduce the temperature needed for free radical initiation.Polymerization retarders can be added to delay gelation so that thepolymer penetrates the formation. Until gelation occurs, the viscosityof the system is like that of water. The gel when formed is extremelyrigid and undergoes no syneresis. With the incorporation of aproprietary stabilizer the gels are stable to about 200° F. This type ofgel is useful in reservoir situations (e.g. temp >140° F.) whereconventional Xanthan gels are not recommended. However, the gelprecurser system will show no selectivity.

Nevertheless, sequential gelation of a combined system is notstraightforward as is shown by the following two baseline experiments.

Baseline Experiment 1. A mixture consisting of 2000 ppm Flocon 4800C(commercial Xanthan biopolymer), 100 ppm NaOH, and 80 ppm Cr⁺³ is madeup in 7% brine and allowed to stand at room temperature. A gel is formedin four hours with the typical consistency of a Flocon 4800C gel, firmbut not rigid, syneresing over time.

Baseline Experiment 2. A mixture consisting of 2000 ppm Flocon 4800C,100 ppm NaOH, 80 ppm Cr⁺³, 100,000 ppm acrylamide, 3600 ppmmethylene-bis-acrylamide, 1200 ppm sodium persulfate, and 562 ppmtriethanol amine is made up in 7% brine and allowed to stand at roomtemperature. A gel is formed in 2-1/2 hours with the typical consistencyof an organic crosslinked polyacrylamide gel, rigid with no syneresis.

The second baseline experiment shows that a free radical polymerizationhas taken place at room temperature, so that the system goes directly tothe organic crosslinked polyacrylalmide gel without the intervention ofthe Flocon 4800C gel. This result probably stems from the redoxinitiator with the Cr⁺³ catalyzing the breaking of the peroxide bond,perhaps assisted by the triethanol amine. The equation for the probablereaction is shown below.

    Cr.sup.+3 +3 S.sub.2 O.sub.8 =+4H.sub.2 O →CrO.sub.4 =+3 SO.sub.4 =+3 SO.sub.4 -+8H.sup.+

For this reason, it is necessary to add a polymerization retarder suchas potassium ferricyanide to obtain sequential gelation. The successfulimplementation of this invention is shown in the following two examples.

EXAMPLE 1

A 200 g mixture was made up of 2000 ppm Flocon 4800C, 100 ppm NaOH, 800ppm Cr⁺³, 100,000 ppm acrylamide, 3600 ppm methylene-bis-acrylamide,2400 ppm sodium persulfate, and 1000 ppm potassium ferricyanide in a 7%brine. The mixture was split in half with each half being placed in ajar. The jars were left left at room temperature for four hours. At theend of that time both jars contained gel. The gels appeared to betypical chromium crosslinked Flocon 4800C gels. Then one jar was placedin a 130° F oven; the other was left on the bench top. The gel in theoven appeared to start firming after one hour. It was left in the ovenovernight. During this time period, it changed to a rigid organiccrosslinked polyacrylamide type gel. The transition time was between 3and 19 hours after the gel was placed in the oven. After five weeks inthe oven at 130° F, the gel is still rigid and has shown no syneresis.The gel left on the bench top at room temperature remained a less firmXanthan-type gel for two weeks with about 20% syneresis. After twoweeks, the gel then changed to a rigid organic crosslinkedpolyacrylamide type gel.

EXAMPLE 2

A 130 g mixture was made up of 3000 ppm Flocon 4800C, 150 ppm NaOH, and120 ppm Cr⁺³ in a 7% brine. The mixture gelled to a typical Flocon 4800C(Xanthan) gel in a little less than four hours. Then 70 g of a mixturecontaining 20 g of acrylamide, 0.72 g methylene-bis-acrylamide, 0.48 gsodium persulfate, and 0.1 g potassium ferricyanide in 7% brine wasblended into the gel. After blending, the concentration of thecomponents is approximately equal to that of Example 1. The mixture wassplit in half with each half being placed in a jar. One jar was put in a130° F oven; the other was left on the bench top. The sample in the ovenchanged to an extremely firm organic crosslinked polyacrylamide type gelovernight. The transition time was between 3 and 19 hours after the gelwas placed in the oven. While it was not immediately as rigid as theorganic crosslinked polyacrylamide gel of Example 1, it achievedcomparable rigidity in a few days. After five weeks in the oven, the gelis still rigid with no syneresis. The gel left on the bench top at roomtemperature remained a less firm Xanthan gel for two weeks with -25%syneresis. The gel then changed to a rigid organic crosslinkedpolyacrylamide type gel.

These examples show that it is possible to form the Xanthan type gel,which gives selectivity, and the organic crosslinked polyacrylamide typegel, which given rigidity and better temperature stability, sequentiallyas would be needed to propagate the gel system selectively in aformation. Furthermore, the presence of Xanthan components does notaffect the desirable characteristics of the organic crosslinkedpolyacrylamide gel. The application of this concept to other polymericgels should be straightforward.

When utilized in the field for profile control purposes, the gelationrate of the system will depend on the amount of the components and thetemperature at which the reaction is conducted. Thus, one can tailor thegel rate and the gel strength of the system by adjusting the amount ofpolymer, the crosslinker, the initiator, the polymerization retarder,pH, and temperature. The higher the temperature at given concentrationsof crosslinker and polymer, the faster the gelation time. If a thickergelled composition is desired, the polymer and crosslinkerconcentrations may be increased for a given temperature.

In preparing the gel system for utilization herein, the aqueous solutioncan comprise fresh water, brackish water, sea water, produced formationwaters and mixtures thereof. A brine comprising sodium chloride in about1 wt. % to 20 wt. % , preferably about 7.0 wt. % can be utilized.Xanthan biopolymer can be used in an amount of from about 1000 to about5000 ppm. Chromic ions utilized should be from about 30 to about 300ppm. Other polyvalent metal ions which can be utilized include aluminum,boron and iron. Alkali metal hydroxides which can be utilized includesodium and potassium hydroxide. Sodium hydroxide is preferred. Theamount of alkali or alkaline earth metal hydroxide utilized should befrom about 10 to about 1000 ppm, preferably about 100 ppm. Acrylamidepolymer used herein should be from about 40,000 to about 200,000 ppm,preferably 100,000 ppm. Methylene-bis-acrylamide should be utilized inan amount from about 500 to about 5000 ppm, preferably about 3,600 ppm.Sodium persulfate can be used in an amount of from about 1000 to about4000 ppm, preferably 2400 ppm. Potassium ferricyanide can be included inan amount of from about 200 to about 2000 ppm, preferably about 1000ppm.

Considerable latitude exists in the design of field processes employingpolymeric slugs containing both selective and non-selective polymers.For example, to minimize any damage to the low permeability zones fromthe non-selective polymer during the initial stages of injection, theconcentration of this component in the slug can be lowered until suchtime as the selective polymer has effectively isolated the lowpermeability zones from further invasion of fluid via filter cakeformation. This is a preferred embodiment of the invention where thepermeability contrast between zones is not large.

Where it is desired to obtain increased sweep efficiency, gels of thisinvention can be used to plug a previously swept portion of a formation.Said gels can be directed to areas of increased porosity by utilizationin any of the below methods.

One method where gels of this invention can be utilized is during awaterflooding process for the recovery of oil from a subterraneanformation. After plugging the more permeable zones of a reservoir withthe sequential gel system of this invention, a waterflooding process canbe commenced. U.S. Pat. No. 4,479,894, issued to Chen et al., describesone such waterflooding process. This patent is hereby incorporated byreference in its entirety.

Steamflood processes which can be utilized when employing the proceduredescribed herein are detailed in U.S. Pat. Nos. 4,489,783 and 3,918,521issued to Shu and Snavely, respectively. These patents are herebyincorporated by reference herein. Of course, for such a situation thein-situ gel must be capable of withstanding steam temperatures.

The sequential gel system described herein can also be used inconjunction with a cyclic carbon dioxide steam stimulation in a heavyoil recovery process to obtain greater sweep efficiency. Cyclic carbondioxide steam stimulation can be commenced after plugging the morepermeable zones of the reservoir with the novel gels of this invention.A suitable process is described in U.S. Pat. No. 4,565,249 which issuedto Pebdani et al. This patent is hereby incorporated by reference in itsentirety. Increased sweep efficiency can be obtained when the subjectgels are used in combination with a carbon dioxide process by loweringthe carbon dioxide minimum miscibility pressure ("MMP") and recoveringoil. Prior to commencement of the carbon dioxide process, the morepermeable zones are plugged with this novel gel system. Carbon dioxideMMP in an oil recovery process is described in U.S. Pat. No. 4,513,821issued to Shu which is hereby incorporated by reference.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A method for sequential gellation in a formationhaving varying permeabilities comprising:(a) making an aqueous solutioncontaining a composition sufficient to form a first gel in an amountsufficient to enter pores in a formation's zone of greater permeabilitywhich gel forms ex-situ and is shear thinning; (b) allowing sufficienttime for the composition to form a gel ex-situ; (c) placing into saidfirst gel an aqueous solution containing a precursor system for a secondgel which forms in-situ and make a gel substantially more resistant toformation conditions than said first gel; and (d) injecting said aqueoussolution containing said first gel into said zone of greaterpermeability where said solution enters the zone of greater permeabilityand subsequentially forms a second solid gel substantially moreresistant to formation conditions than said first gel.
 2. The method asrecited in claim 1 where said first gel forms in about four hours atambient temperature so as to make a firm but not rigid gel whichwithstands formation temperatures of up to about 150° F.
 3. The methodas recited in claim 1 where said second gel forms a substantially rigidgel which undergoes substantially no syneresis and which can withstandformation temperatures up to about 200° F.
 4. The method as recited inclaim 1 where said composition contains Xanthan biopolymer in an amountof from about 1,000 to about 5,000 ppm by weight, 30 to about 300 ppm ofchromic ions, and 10 to about 1,000 ppm of NaOH where said aqueoussolution contains about 1 to about 20 wt. % NaCl.
 5. The method asrecited in claim 1 where said precursor system is a mixture ofacrylamide and methylene-bis-acrylamide in an amount of from about40,000 to about 200,000 ppm acrylamide, from about 500 to about 5,000ppm methylene-bis-acrylamide, about 1,000 to about 4,000 ppm sodiumpersulfate, about 200 to about 2,000 ppm potassium ferricyanide wheresaid solution contains about 1 to about 20 wt. % NaCl.
 6. The method asrecited in claim 1 where a water flood, a steam flood, or a carbondioxide flood is directed into a zone of lesser permeability followingstep c).
 7. The method as recited in claim 1 where a polymerizationretarder is placed into said aqueous solution so as to allow propagationto a desired distance into said more permeable formation zone.
 8. Amethod for gellation of a formation having zones of varying permeabilitycomprising:(a) placing into an aqueous solution a first compositionsufficient to form ex-situ a size selective, shear thinning first gelsufficient to enter pores in a formation's greater permeability zonewhere it reheals, (b) placing into said aqueous solution a secondcomposition sufficient to form a second in-situ gel which issubstantially more resistant to formation conditions than said firstgel; (c) allowing said aqueous solution sufficient time to form theex-situ gel; and (d) injecting said aqueous solution containing said gelinto the formation where it enters the zone of greater permeability andthereafter forms a second solid gel substantially more resistant toformation conditions than said first gel.
 9. The method as recited inclaim 8 wherein said composition contains Xanthan biopolymer in anamount of from about 1,000 to about 5,000 ppm by weight, 30 to about 300ppm of chronic ions, and 10 to about 1,000 ppm of NaOH where saidaqueous solution contains about 1 to about 20 wt. % NaCl.
 10. The methodas recited in claim 8 where said second composition is a mixture ofacrylamide and methylene-bis-crylaimde in an amount of from about 40,000to about 200,000 ppm acrylamide, from about 500 to about 5,000 ppmmethylene-bis-acrylamide, about 1,000 to about 4,000 ppm sodiumpersulfate, about 200 to about 2,000 ppm potassium ferricyanide wheresaid solution contains about 1 to about 20 wt. % NaCl.
 11. The method asrecited in claim 8 where said first gel forms in about four hours atambient temperature so as to make a firm but not rigid gel whichwithstands formation temperatures of up to about 150° F.
 12. The methodas recited in claim 8 where said second gel forms a substantially rigidgel which undergoes substantially no syneresis and which can withstandformation temperatures up to about 200° F.
 13. The method as recited inclaim 8 where a water flood, a steam flood, or a carbon dioxide flood isdirected into a zone of lesser permeability following step (d).
 14. Amethod for gellation of a formation having zones of varying permeabilitycomprising:(a) placing into an aqueous solution a first compositionsufficient to form ex-situ a size selective shear thinning first gelwhich comprises(i) a Xanthan biopolymer; (ii) sodium hydroxide, and(iii) a polyvalent metal ion (b) placing into said aqueous solution asecond composition sufficient to from a second in-situ gel which issubstantially more resistant to formation conditions than said first gelwhich comprises(i) a polyacrylamide polymer, (ii) an organiccross-linker, (iii) an initiator, and (iv) a polymerization retarder (c)allowing said aqueous solution sufficient time to form the ex-situ gel;and (d) injecting said aqueous solution containing said gel into theformation where it enters a zone of greater permeability, reheals andthereafter forms a solid gel substantially more resistant to formationconditions than said first gel.
 15. The method as recited in claim 14where said first composition contains Xanthan biopolymer in an amount offrom about 1,000 to about 5,000 ppm by weight, 30 to about 300 ppm ofchromic ions, and 10 to about 1,000 ppm of NaOH where said aqueoussolution contains about 1 to about 20 wt. % NaCl.
 16. The method asrecited in claim 14 where said second composition comprises about 40,000to about 200,000 ppm acrylamide, from about 500 to about 5,000 ppmmethylene-bis-acrylamide, as the organic cross-linker, about 1,000 toabout 4,000 ppm sodium persulfate, as the initiator, about 200 to about2,000 ppm potassium ferricyanide, as the retarder where said solutioncontains about 1 to about 20 wt. % NaCl.
 17. The method as recited inclaim 14 where said first gel forms in about four hours at ambienttemperature so as to make a firm but not rigid gel which withstandsformation temperatures of up to about 150° F.
 18. The method as recitedin claim 14 where said second gel forms a substantially rigid gel whichundergoes substantially no syneresis and which can withstand formationtemperatures up to about 200° F.
 19. The method as recited in claim 14where a water flood, a steam flood, or a carbon dioxide flood isdirected into a zone of lesser permeability following step (d).
 20. Themethod as recited in claim 14 where gel rate and gel strength arecontrolled by adjusting the amount of polymer, the cross-linker, aninitiator, a polymerization retarder, pH, and temperature.
 21. Themethod as recited in claim 14 where polymer contained in said secondcomposition is lowered while initially injecting the aqueous solutioninto the formation to minimize damage to a zone of lower permeability insaid formation.